1. Field of the Invention
The present invention relates to a constant voltage circuit which is preferably used in an infrared remote control receiver, a low-frequency highly sensitive sensor circuit and the like, and to an infrared remote control receiver equipped with the same, and especially relates to countermeasures against power source noise thereof.
2. Description of the Related Art
FIG. 9 is a block diagram which entirely shows an example of a reception system of an infrared remote control receiver 1, and FIGS. 10A to 10D are waveform diagrams of individual portions thereof. This receiver 1 converts infrared transmission code signals to photocurrent signals Iin shown in FIG. 10A in an external photodiode 2 to input the signals into a reception chip 3 which is configured as an integrated circuit, and outputs output signals OUT shown in FIG. 10D demodulated in the reception chip 3 to a microcomputer which controls electronics and the like. The infrared signals are ASK signals which are modulated by a predetermined carrier from 30 kHz to 60 kHz approximately.
Within the reception chip 3, the photocurrent signals Iin shown in FIG. 10A are amplified sequentially in a first amplifier (HA) 4, a second amplifier (2nd AMP) 5 and a third amplifier (3rd AMP) 6, and a carrier component as shown by a reference numeral xcex1l in FIG. 10B is taken out in a band pass filter (BPF) 7 which is matched with the frequency of the carrier. Then, the carrier component is detected at a carrier detection level Det denoted by a reference numeral xcex12 in a detector circuit 8 of the following stage, a time when the carrier exists is integrated in an integration circuit 9 as shown by a reference numeral xcex111 in FIG. 10C, and an integration output Int obtained thereby is compared with a predetermined discrimination level denoted by a reference numeral xcex112 in a hysteresis comparator 10, whereby the presence or absence of the carrier is recognized, and the signals are digitally outputted as output signals OUT shown in FIG. 10D.
A low pass filter 11 is placed on the output side of the first amplifier 4, by which a direct current level by a fluorescent lamp and sunlight is detected, and in the second amplifier 5 of the following stage, the part of the direct current level is eliminated from a direct output of the first amplifier 4 and the output is amplified, whereby the influence of noise of the fluorescent lamp, sunlight and so on is removed at a certain level. Moreover, a ABCC (Auto Bias Current Control) circuit 12 is placed with reference to the first amplifier 4, and by this ABCC circuit 12, a direct current bias of the first amplifier 4 is controlled in response to an output of the low pass filter 11.
It has been mainstream up to now that a power supply voltage of the infrared remote control receiver 1 configured in the above manner and a highly sensitive sensor circuit is a 5 V system. However, in recent years, a power supply voltage of a peripheral LSI has been decreased to, for example, 3 V, and power consumption thereof has been decreased, and also regarding the infrared remote control receiver 1 and a highly sensitive sensor circuit, voltage decrease has been strongly desired. On the other hand, a request of a device supplier for a power supply voltage has a broad range. For example, a guarantee of a minimum operating voltage of 3.3 Vxc2x10.3 V is required in one system, and 2.4 V or 1.8 V is required in another system using a battery. In this manner, regarding voltage decrease, a response to a broad range of power supply voltage is often required in one device.
With reference to such a response as described above, power source noise is one of the design problems to take countermeasures. The power source noise enters from a power source in most cases and enters from a load side in some cases, thereby causing jitters in a power supply voltage. In the infrared remote control receiver 1 and a highly sensitive sensor circuit, an amplifier (denoted by reference numerals 4, 5 in FIG. 9) amplifies infrared signals and sensor signals at a very high gain, and therefore the amplifier is very apt to be affected by power source noise. In a case where power source noise influences the operation of the amplifier in the circuit, it is amplified to cause malfunction throughout.
Although, for this reason, it has been historically recommended to insert and mount a noise filter in a power supply line of a sensor circuit and the like, states of power source noise are different depending on used sets, and trouble is often caused. Furthermore, because of downsizing of a package in recent years, it becomes difficult to mount such a power source filter resistor and a capacitor in a package, so that there is no other choice to build a constant voltage circuit for countermeasures against power source noise in an integrated circuit.
FIG. 11 is a view for describing countermeasures against power source noise of a typical prior art. In this prior art, by inserting a constant voltage circuit 22 in a power supply bias of an amplifier 21, power source noise is decreased. The constant voltage circuit 22 is a so-called three-terminal regulator. A direct current output voltage Vs from the constant voltage circuit 22 is fixed, and by preventing variation of a power supply voltage Vcc, that is, preventing the power source noise from transmitting to the output voltage Vs, the influence of the power source noise on the amplifier 21 is prevented or decreased.
Here, in a case where the voltage range of the power supply voltage Vcc required to be responded is broad as described before, it is necessary to set the value of the output voltage Vs of the constant voltage circuit 22 in relation to the minimum voltage that guarantees the operation. As a result, the operation range of the amplifier 21 is also restricted by the voltage. In other words, even in a case where the amplifier is used in a state that the power supply voltage Vcc is not the minimum voltage that guarantees the operation, for example, even in a case where the amplifier is used at 3.3 V while the minimum operation voltage thereof is 2.4 V, the output voltage Vs of the constant voltage circuit 22 remains set to less than 2.4 V, so that the maximum output amplitude from the amplifier 21 does not become 3.3 V but remains 2.4 V.
As a general example of countermeasures against such a problem, a configuration shown in FIG. 12 which is another prior art may be cited. In this prior art, the power supply voltage Vcc is supplied to the amplifier 21 via a NPN transistor q, and the power supply voltage Vcc is supplied to a base of the transistor q via a low pass filter constituted by a resistor r and a capacitor c. Therefore, power source noise is decreased in the low pass filter, and a current capacity is ensured in the transistor q to become a bias voltage (Vs) of the amplifier 21, whereby the countermeasures against the power source noise are taken. Since the bias voltage (Vs) varies in conjunction with the power supply voltage Vcc, the operation range of the amplifier 21 can be enlarged when the power supply voltage Vcc is high.
However, according to the prior art described above, there is a problem that, since the infrared remote control receiver 1 and a sensor circuit that handle low-frequency signals of tens of kHz or so require that a time constant of RC is set to a large value, it is impossible to achieve integration with ease. For example, a capacity value which allows integration is normally 100 pF or less. Furthermore, a practical capacity value for decreasing the influence on a chip area is 20 pF or so. In order to achieve a capability of removing power source noise to some extent while using this capacity value, a large time constant by an enormously large resistance component is needed. For example, in a case where it is required to set a power source noise removing rate PSRR at 40 kHz to xe2x88x9240 dB (1/100), assuming that c=20 pF, the resistance value R of the resistor r is obtained by expressions shown below:                     PSRR        =                  1                                    1              +                                                (                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    fCR                                    )                                2                                                                        (        1        )            
Therefore,                     R        =                                                            1                                                      (                                          2                      ⁢                                              xe2x80x83                                            ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      fC                                        )                                    2                                            ⁡                              [                                                      1                                          PSRR                      2                                                        -                  1                                ]                                              ≈                      19.9            ⁢                          xe2x80x83                        ⁢            M            ⁢                          xe2x80x83                        ⁢            Ω                                              (        2        )            
Accordingly, it is difficult to place a resistance value of this order as it is in the integrated circuit.
Further, according to the prior art described above, there is also a problem that, since an operating voltage (VBE) of the transistor q is needed, a value of Vxcex1 which is difference between Vcc and Vs becomes large and an operating voltage of the amplifier 21 does not become so large.
An object of the present invention is to provide a constant voltage circuit which has a configuration allowing integration and which can ensure an operating voltage of a load side in conjunction with a power supply voltage, and provide an infrared remote control receiver using the same.
The invention provides a constant voltage circuit which removes power source noise by outputting a direct current constant voltage responsive to a direct current input power supply voltage,
the constant voltage circuit comprising:
a direct current level shift circuit for effecting a shift from the input power supply voltage by a predetermined direct current voltage level;
a power source noise removing circuit including a transconductance amplifier, for removing the power source noise from an output of the direct current level shift circuit; and
a PNP type transistor interposed in series with a power supply line between input and output terminals, a base of the PNP type transistor being driven by an output from the power source noise removing circuit.
According to the invention, a direct current input power supply voltage is outputted to the load side via the PNP type transistor in which difference between emitter and collector voltages, that is, input and output voltages is small, and the base thereof is driven by a base current from which power source noise is removed in the power source noise removing circuit. Then, an input to the power source noise removing circuit is produced by shifting a level from the side of the input power supply voltage in the direct current level shift circuit.
Therefore, an output voltage varies in response to a direct current input power supply voltage, and a voltage drop from the input current voltage is relatively low owing to the PNP type transistor, with the result that an operation voltage on the load side can be ensured. Moreover, since the power source noise removing circuit includes the transconductance amplifier, it is possible by setting transconductance gm of a time constant C/gm to a small value to obtain a capacity C of a value which allows integration, in order to increase a power source noise removing rate at low frequencies.
Furthermore, in the invention it is preferable that a level shift amount in the direct current level shift circuit is set to around a collector-emitter saturation voltage of the PNP type transistor.
According to the invention, an output voltage can be maximized to direct current variation of the power supply voltage, so that it is possible to set a direct current operation range of a load side circuit to a maximum value while removing power source noise sufficiently.
Still further, in the invention it is preferable that:
an input circuit of the transconductance amplifier constituting the power source noise removing circuit is provided with first to fourth transistors QN1 to QN4 of a same conducting type and a resistor R1;
bases or gates of the first and second transistors QN1, QN2 are connected to each other to become a first input terminal of the transconductance amplifier, and emitters or sources of the first and second transistors QN1, QN2 are connected to a first constant current source F1 in common;
bases or gates of the third and fourth transistors QN3, QN4 are connected to each other to become a second input terminal of the transconductance amplifier, and emitters or sources of the third and fourth transistors QN3, QN4 are connected to a second constant current source F2 in common;
the emitters or sources of the first and second transistors QN1, QN2 are connected to the emitters or sources of the third and fourth transistors QN3, QN4 via the resistor R1; and
collectors or drains of the first and fourth transistors QN1, QN4 are connected to a power source terminal.
According to the invention, even in the case of setting a capacity of the resistor R1 to a value which allows integration into an integrated circuit, it is possible to produce very low transconductance gm and obtain a sufficient noise removing rate.
Still further, in the invention it is preferable that:
an output circuit of the transconductance amplifier constituting the power source noise removing circuit is provided with fifth and sixth transistors QP5, QN5 of different conducting types from each other;
a base or gate of the fifth transistor QP5 is connected to a base or gate of the sixth transistor QN5; and
a capacity C of the transconductance amplifier is charged and discharged by a base or gate current io.
According to the invention, the base or gate current io of the fifth and sixth transistors QP5, QN5 is used to produce sufficiently small transconductance gm, and a low pass filter is implemented, so that even in the case of setting the capacity C to a value which allows integration, it is possible to obtain a large time constant responsive to low frequency signals.
Still further, in the invention it is preferable that:
the output circuit of the transconductance amplifier constituting the power source noise removing circuit is further provided with, in correspondence with the fifth and sixth transistors, seventh and eighth transistors QP6, QN6 of different conducting types from each other;
the fifth transistor QP5 of one conducting type is paired with the sixth transistor QN5 of the other conducting type, and the seventh transistor QP6 of one conducting type is paired with the eighth transistor QN6 of another conducting type;
a base or gate of the seventh transistor QP6 is connected to a base or gate of the eighth transistor QN6, collectors or drains of the fifth and seventh transistors QP5, QP6 are connected to a ground (GND) or a power source in common, a collector or drain of the sixth transistor QN5 is connected to the power source or the GND, an emitter or source of the sixth transistor QNS is connected to a collector or drain of the eighth transistor QN6, and an emitter or source of the eighth transistor QN6 is connected to the GND or the power source; and
a differential current is inputted from the input circuit to emitters or sources of the fifth and seventh transistors QP5, QP6.
According to the invention, by forming the input circuit so as to have a differential configuration, it is possible to decrease power source noise affecting the power source noise removing circuit itself, and even when a parasitic photocurrent is generated in a base or gate terminal of the PNP type transistor, the photocurrent is cancelled, and it is possible to prevent transconductance gm from varying.
Still further, in the invention it is preferable that of the fifth to eighth transistors QP5, QN5, QP6, QN6, transistors which use a minute base or gate current io are PNP type transistors QP5, QP6, which transistors QP5, QP6 are formed so as to be of a lateral structure, and with respect to the PNP type transistors QP5, QP6, a parasitic photocurrent compensating circuit is disposed.
According to the invention, it is possible to cancel a parasitic photocurrent which is generated in the case where, of the fifth to eighth transistors QP5, QN5, QP6, QN6, the PNP type transistors QP5, QP6 using a minute base or gate current io are of a lateral structure allowing easy production without using a special process, by the use of the parasitic photocurrent compensating circuit. With this, it is possible to suppress variation of transconductance gm.
Still further, in the invention it is preferable that of the fifth to eighth transistors QP5, QN5, QP6, QN6, transistors which use a minute base or gate current io are PNP type transistors QP5, QP6, which PNP type transistors QP5, QP6 are formed so as to be of a vertical structure.
According to the invention, it is possible to decrease the parasitic photocurrent itself.
Still further, in the invention it is preferable that a voltage is supplied to the collector of at least one of the fifth and seventh transistors QP5, QP6, and collector-emitter voltages of the transistors are set to a substantially equal value.
According to the invention, the imbalance due to Early effect between the fifth and seventh transistors QP5, QP6 can be reduced, and an offset of a direct current voltage can be decreased.
Still further, in the invention it is preferable that an input of a first buffer circuit is connected to the base or gate of at least one of the fifth and seventh transistor QP5, QP6, and an output of the buffer circuit is connected to the collector or drain of the aforementioned transistor.
Still further, in the invention it is preferable that an input of a first buffer circuit is connected to the base or gate of at least one of the fifth and seventh transistors QP5, QP6, a level adjusting circuit which shifts a direct current level is added to an output of the first buffer circuit, an input of a second buffer circuit is connected to an output of the level adjusting circuit, and an output of the second buffer circuit is connected to the collector or drain of at least one of the fifth and seventh transistors QP5, QP6.
According to the invention, collector-emitter voltages of the fifth and seventh transistors QP5, QP6 are set so as to become constant to variation of a power supply voltage, whereby the imbalance due to Early effect of the respective transistors QP5, QP6 can be reduced, and an offset of a direct current voltage can be decreased.
Still further, the invention provides an infrared remote control receiver comprising any one of the constant voltage circuits described above.
According to the invention, the infrared remote control receiver is very apt to be affected by power source noise because an amplifier which is a load circuit handles low-frequency signals and a gain thereof is high, and therefore able to preferably use the constant voltage circuit described above.